1. Field of the Invention
The present invention relates to an SiC-based pressureless sintered product having high toughness and high strength.
2. Description of the Related Art
An SiC-based sintered product has good characteristics in, e.g., an oxidation resistance, a corrosion resistance, and a thermal shock resistance and is expected to be a high-temperature structural material for use in gas turbine parts, a high-temperature heat exchanger, and the like. As a result of recent studies of such an SiC-based sintered product, it is found that the simultaneous addition of B and C elements is particularly effective, and various types of high-density SiC-based sintered products have been realized by using a sintering assistant of this type. Although various types of improvements and modifications in characteristics have been made to put SiC-based sintered products into practical use as described above, toughness of such SiC-based sintered products has not been improved yet.
A large number of methods of solving this problem have been reported so far. For example, each of Published Unexamined Japanese Patent Application Nos. 64-87562, 64-87563, and 64-87564 discloses a nonoxide-based composite sintered product formed by combining a transition metal carbide or boride such as TaC, NbB.sub.2, or VB.sub.2 as a second phase component in SiC. According to these publications, a powder of a second phase component as described above having an average grain size of 3 to 8 .mu.m is dispersed in an SiC powder and the mixture is hot-pressed to form a high-density composite sintered product having a high toughness value. Although the toughness of this composite sintered product is improved, however, its fracture strength is still insufficient and largely reduced at high temperatures. In addition, since this sintered product is a hot-pressed product having a limited shape, it has not been satisfactorily practical. Although many other attempts have been made to improve the toughness of SiC-based sintered products, they all have both advantages and disadvantages, and the above conventional problem of low toughness has not been solved yet.
As described above, the problem of low toughness of the conventional SiC-based sintered product prevents its practical use, and many methods of improving the toughness have been reported. According to these methods, however, although the toughness of the sintered product is improved, another problem of low strength or the like arises. As a result, no method of obtaining high toughness in an SiC-based sintered product without degrading its various characteristics has been achieved.